\section{Conclusion}
\label{sec:conc}
In this paper, we identify the virtual network diagnosis problem and 
articulate the challenges involved. We propose APLAD to overcome these
challenges, which is a framework that allows a cloud
provider to offer a sophisticated virtual network diagnosis service
to its tenants. Our evaluation shows that by co-locating
flow capture points and table servers, APLAD can capture tenant's
traffic flows without impacting their performance, and the network
diagnosis query can be executed quickly on distributed tables in
response to tenants' requests without introducing too much extra
network traffic. This architecture scales to the size of a real data center 
network. To the best of our
knowledge, ours is the first attempt at addressing the virtual network
diagnosis problem, and APLAD is a feasible and useful solution.

